1. Field of the Invention
The present invention relates to reducing carbon induced damage to resistance welded metal composites. More particularly, the present invention relates to sound damping, weldable, laminated metal composites possessing minimal alloying with carbon and/or carbide formation in the vicinity of welds from resistance spot welding.
2. Discussion of the Related Art
Metal composites are used to reduce noise and vibration in a wide range of applications. Such applications include automobiles or other vehicles, machinery, appliances, power equipment and the like. These metal composites typically include a viscoelastic layer disposed between (sandwiched by) two metal sheets.
In order to facilitate electrical conduction from the metal sheets and through the composite during the welding process, preferably, the viscoelastic layer is doped with electrically conductive particles. During resistance welding, the current flow from the welding electrode through the entrained, conductive particles generates sufficient heat at the weld zone to melt the conductive particles. Because the viscoelastic layer typically constitutes a polymeric mixture, the melting of conductive particles generates thermal gradients, causing discrete evaporation, and/or creating carbon residue. The molten particles in the viscoelastic layer may alloy directly with adjacent metal (primary alloys) or may first combine with other residues/thermal decomposition products from the heated viscoelastic material to then alloy with the metal (secondary alloys).
In the context of localized decomposition products of the viscoelastic layer around the conductive particles melt zone, elemental carbon is a particularly undesirable impurity. It is well known that carbon aggressively alloys with ferrous metals to reduce their melting points and/or form carbon-rich hard areas, and titanium to form carbides. In addition to the adverse metallurgical aspects on weld quality, the physical aspects of melting and local vaporization of the viscoelastic layer cause localized deterioration. Gas at high internal pressure is generated at or near the weld site. These physical anomalies occur in precisely the vicinity of undesirable selective alloying/metallurgical imperfections. The foregoing results not only in metallurgically degraded welds but also physically degraded structures, e.g., blisters or blowholes that adversely impact the composite's structural integrity.
Testing on low carbon steel composites has shown that melting/liquefied conductive particles, particularly those with significant presence of iron or nickel, will absorb or adsorb carbon from the decomposed viscoelastic layer. The resulting enriched carbon-containing compositions promote carbon diffusion into the metal sheets lowering the sheet's melting point and/or forming hard carbon rich areas including carbides. Consequently, the final welded region of the composite includes imperfections, inconsistencies, and other deleterious non-uniformities.
In view of the foregoing problems, it is clear that improvements can be made to the prior art.